1. Field of the Invention
The present invention relates to monitoring the contamination of parts such as substrate wafers in semiconductor and microelectromechanical system (MEMS) fabrication processes.
2. Description of the Prior Art
The ever decreasing size of the active components of semiconductors and microelectromechanical systems make them increasingly sensitive to the presence on their surface or within their mass of foreign bodies that are generally conveyed by the atmosphere that surrounds the substrate wafers during processing. Consequently, the presence of foreign bodies, usually referred to as contamination, must be reduced as much as possible during semiconductor and microelectromechanical system fabrication processes.
For this reason, the fabrication processes are carried out in white rooms where the atmosphere is maintained at a very low level of contamination. The level of contamination of the atmosphere in the white room is monitored continuously.
For example, the document US-2004/0023419 describes a method of measuring the contamination adsorbed on optical surfaces intended for photolithographic processes. The method utilizes a polymer that has a high capacity for adsorbing the compounds liable to constitute gaseous contamination. The polymer is then analyzed using thermal desorption, gas-phase chromatography and mass spectrometry. The polymer should preferably remain in the atmosphere to be monitored for a sufficient time, of the order of a few hours to a few days or a few weeks, to enable the collection of a sufficient mass of contaminants present in the atmosphere at a relatively low concentration. The drawback of this is that the measurement is not effected in “real time” and there is no provision for a tailored and rapid reaction in the event of gaseous contamination at a low concentration.
The document DE-101 33 520 describes another system that determines the properties of the atmosphere in a white room used for the fabrication of semiconductors. The system is incorporated into a standard front-opening transport enclosure (FOUP) so as to have the usual overall dimensions of the transport enclosures that contain the substrate wafers during fabrication. The transport enclosure containing the monitoring system aspirates air from the white room, analyzes it, and returns it to the white room, thereby monitoring the atmosphere in the white room along the route taken by the standard transport enclosures (FOUP) containing the substrate wafers. The parameters measured may be the air temperature in the white room, the relative humidity, the level of particulate pollution and the level of pollution by certain unwanted gases. The measuring system must be adapted to have an overall size less than the interior space available in a standard front-opening transport enclosure (FOUP).
Because of this small size, the measuring devices are unable to measure very low levels of gaseous contamination present in trace amounts of the order of 1 ppb (part per billion).
What is more, the devices cited above do not diagnose the atmosphere contained in the substrate wafer transport enclosures themselves, the measurements being effected globally on the atmosphere of the white room containing the substrate wafer transport enclosures.
Now, these substrate transport enclosures are porous environments that concentrate contaminants and in particular organic, amine and acid contaminants. The substrate wafers are continually exposed to these atmospheres while awaiting processing. After these semiconductor fabrication processes, the substrate wafers are charged with process gases which escape from the substrate wafers and contaminate the walls of the transport enclosures. Such contamination is very harmful to semiconductor substrates such as wafers or masks.
If is found that the contamination monitoring methods known in the art are inadequate, in particular in the case of low level gaseous contamination of substrate wafers or other parts. Such low level molecular gaseous contamination by substitution, addition or doping of the surface of the substrate wafers can create defects that may not be visible using current post-production measurement techniques but will reduce the service life of semiconductors and electromechanical microsystems fabricated in this way.
Low level molecular contamination by substitution, addition or doping of the substrates is a real problem. Semiconductors produced in this way may be declared satisfactory on leaving the plant but it is found that their service life may be shortened. There is therefore a great benefit to be obtained by reducing this low level molecular contamination.
The problem addressed by the present invention is that of reducing further the risks of defects appearing in semiconductors and electromechanical microsystems fabricated by fabrication processes including steps of transporting and/or storing substrate wafers in transport enclosures.
At the same time, the invention aims to avoid all risk of additional contamination of the substrates by adding materials such as gaseous adsorption polymers.
The invention further aims to reduce the risk of transmission of contamination by the substrate wafers or other parts contained in the transport enclosures.
The basic idea of the invention is to monitor the atmosphere in the transport enclosures themselves using powerful external analysis means communicating with the internal atmosphere of the transport enclosures.
As a result, the analysis means can be of a size that is independent of, and in particular greater than, the interior volume of the standard transport enclosures and the isolation of substrate wafers contained in the standard transport enclosure is maintained during analysis.
It is therefore possible to use analysis means capable of providing an analysis in real time, i.e. in a very short time, and of detecting and measuring gaseous contamination at very low concentrations, of the order of 1 ppb.